In the latest automatic two-wheeled vehicle, an apparatus for adjusting a height of the vehicle has often been provided so that rider's feet cannot only reach the ground to stop the vehicle when running on the urban area but also a performance of the vehicle as covering a wasteland may be improved. There have conventionally been two kinds of a rear cushion type apparatus: one is a type in which an initially set load can be changed and the other to a type in which its entire length can be changed.
A rear cushion having an apparatus for adjusting the height of vehicle, in which the initially set load can be changed, is known from, for example, U.S. Pat. No. 4,159,105. Its schematic views are shown in FIGS. 10 to 12, in which reference numeral 1 is a damper cylinder, 2 a piston, 3 a piston rod, 4 a suspension spring, 5 a rebound spring, 6 a stopper rubber, 7 a member for mounting the piston rod 3, around which a cylinder 11 for adjusting the height of vehicle is formed as one member for adjusting the height of vehicle. 8 is a mounting member used for either one of the vehicle body and axle, 9 the other remaining mounting member used for the vehicle body or axle, 10 a piston for adjusting the height of vehicle, which is arranged as the other member for adjusting the same, the piston vertically sliding within the above-mentioned adjusting cylinder, with one end of the suspension spring supported thereagainst. FIG. 10 relates to a state in which the suspension spring 4 is set to be slightly compressed in an initially loaded state, i.e., when the piston rod 3 is withdrawn to maximum extent out of the cylinder until the rebound spring 5 is completely compressed, the state being one in which the shock absorber is extended to maximum extent. FIG. 11 illustrates a state in which since a rider rides on the vehicle the suspension spring 4 is compressed from a position of FIG. 10 and a mounting length is shortened and yet the height of vehicle is not adjusted yet, the state being suitable for running on the urban area because his feet can reach the ground. FIG. 12 illustrates a state in which when the rider rides on the vehicle and runs into a wasteland a hydraulic oil is supplied into the cylinder for adjusting the height of vehicle formed about the member 7 for mounting the piston rod to outwardly displace the member 7 with the result that the height of vehicle is increased from L.sub.1 to L.sub.2. l.sub.t is a total stroke, l.sub.1 a compession stroke, and l.sub.2 a rebound stroke. (Actually, although the stroke 1.sub.t equals a value obtained by deducting a compressed length of the rebound spring and the basic lengthes assumed by, for example, an oil seal and rod guide and the like at the end of a bore of the cylinder, in this case, it is represented as such including them for convenience of description. Unless otherwise specified, the same applies to the following part of this specification.)
If a displacement vs. load curve of the suspension spring is graphically plotted in which an initial load is set, then K.sub.1 is obtained as in FIG. 13. I.sub.1 is the value of the initial load. If the initiallly set load is varied from I.sub.1 to I.sub.2, then the displacement vs. load curve is plotted as K.sub.2. If the piston 10 for adjusting the height of vehicle is now upwardly displaced from a position of the lower end within the cylinder of FIG. 11 to increase the initially set load of the suspension spring, then, since the load applied to the suspension spring is the same as before adjusting the height of vehicle, the piston rod is withdrawn by that displacement of the piston 10 from a damper cylinder 1 to increase the height. Consequently, the rider will feel rather stiff to ride on the vehicle because the initiallly set load of the suspension spring is increased. Besides, since the total stroke l.sub.t of the hydraulic shock absorber remains the same before (FIG. 11) and after (FIG. 12) the height is increased, even if he increases the height of vehicle after he runs into the wasteland, a great shock cannot be absorbed. Further, although the total stroke l.sub.t remains the same before and after the height is increased, the position of the piston in 1G state is changed and the distributions of stroke at the compression and extension sides become different with each other. As a result, they respectively assume a relationship of the strokes: compression side stroke l.sub.1 ' and the rebound stroke l.sub.2 ', and when the height of vehicle is increased, then the rebound stroke is shortened, which causes a frequency at which the piston strikes against the rebound spring to be increased with the result that a proper cushioning performance cannot be achieved.
Further, another hydraulic shock absorber having a height adjusting apparatus of a type in which a full length may be changed is known from. for example, Japanese Patent Laid-open 79776/191980. Its schematic views are shown in FIGS. 14 and 15. 21 is a damper cylinder, 22 a piston, 23 a piston rod, 24 a suspension spring, 25 a rebound spring, 26 a rubber stop, 27 a spring bearing rigidly fixed along the outer periphery of the piston rod, 28 a mounting member for either side of the body and axle of a two-wheeled vehicle, 29 a mounting member for the other side of the body or axle, 30 a piston for adjusting the height of vehicle serving as one member for adjusting the same, which is fixed to the upper end of the piston rod, 31 a cylinder for adjusting the height of vehicle, which is arranged as the other member for adjusting the same and 32 a spring bearing fixed to the outer periphery of the cylinder. When, in a state prior to increase of the height of vehicle, in which a rider rides, as shown in FIG. 14, a hydraulic oil is supplied to the upper port of the cylinder 31, assuming for ease of understanding that the piston rod 23, hence, the piston 30 stands still and only the cylinder 31 is elevated, the height of vehicle is increased as shown in FIG. 15, so that the mounting length is increased from L.sub.1 to L.sub.2. As a result, since the initially set load of the suspension spring remains the same, when the height of the vehicle is increased, the rider can experience the same feeling as when he runs on the flat urban area, but since the total stroke l.sub.t remains the same, a possible major thrust from below, which can often be experienced during running on the wasteland, cannot be absorbed, as in the case of the above-described shock absorber in which the initially set load can be changed. Besides, since the mounting length is made longer by a stroke of the piston 30 as the shock absorber is compressed to maximum extent after the height of vehicle is increased, a large space becomes necessary between the body of vehicle and the axle.
Also, in the conventional apparatus for adjusting the height of vehicle for use in a front fork of the two-wheeled vehicle, there are two types: a type in which the initially set load can be changed and a type in which the entire length can be changed.
The front fork of a type in which the initially set load may be changed is known from, for example, Japanese Utility Model 160890/1983, which is illustrated in FIGS. 16 to 18. In the same figures, 41 denotes an inner barrel, 42 an outer barrel, 43 a sheet pipe, 44 a suspension spring, 45 a rebound spring, 46 a piston for adjusting the height of vehicle and 47 a cylinder for adjusting the height of vehicle. FIG. 16 shows a state in which an initially set load is set, i.e., a state in which the suspension spring 45 is slightly compressed with the height of vehicle not adjusted as the inner barrel is withdrawn to maximum extent and the rebound spring 45 is completely compressed. FIG. 17 shows a state in which when a rider rides on a two-wheeled vehicle having a front fork of FIG. 16 and the suspension spring is compressed the height of vehicle is not adjusted, which is suitable for running on a flat urban area. FIG. 18 shows a state in which as a rider rides on a two-wheeled vehicle and runs on a wasteland a hydraulic oil is supplied to a cylinder for adjusting the height of vehicle 47 formed at a fork bolt 48 of an inner barrel 41 to displace the inner barrel 41 with respect to an outer barrel 42 with the result that the height of vehicle is increased from L.sub.1 to L.sub.2. l.sub.t is a total stroke. Actually, its value becomes equal to a value obtained by deducting a basic lengh assumed by, for example, an oil lock collar at the tip end of the inner barrel and the compressed lengthes of the rebound spring and the like. However, in this embodiment, it is generally represented as l.sub.t for convenience of discription. Unless otherwise specified, the same also applies to the following part of specification.
If a displacement vs. load curve of the suspension spring is graphically plotted as the initial load of FIG. 16 is set, then K.sub.1 is obtained as seen in FIG. 19. I.sub.1 is the value of the initially set load. If the initially set load is changed from I.sub.1 to I.sub.2, then the displacement vs. load curve of the suspension spring is shifted to K.sub.2. Now let us assume that in order to increase the height of the vehicle the piston 46 of FIG. 17 is downwardly displaced from the upper end position within the cylinder 47 to increase the initial load imposed on the suspension spring 44, then, since the load imposed thereon is the same as before adjusting the height of vehicle, the inner barrel is withdrawn by a displacement of the piston 46 from the outer barrel with the result that the height of the vehicle is increased. In consequence, the rider can feel rather stiff to ride on the vehicle by such an increased initial load of the suspension spring.
Besides, since the total strokes l.sub.t of the hydraulic shock absorber are the same before (FIG. 17) and after (FIG. 18) the height of vehicle is increased, even if he runs into the wasteland and accordingly increases the height of vehicle, it cannot absorb large shocks. In addition, although the total strokes l.sub.t remain the same before and after the height of vehicle is increased, the position of the stroke of the piston in the condition of 1G is altered, so that the distributions of the stroke become different at the compression and extension sides, and when the height of vehicle is increased as in FIG. 18, then the rebound stroke l.sub.2 ' is shortened, which causes a frequency at which the piston strikes against the rebound spring to be increased with the result that a proper cushioning performance cannot be achieved.
Further, a front fork provided with an apparatus for adjusting the height of vehicle of a type in which the entire length can be changed is known from, for example, Japanese Patent Laid-open 23036/1988. Its schematic views are shown in FIGS. 20 and 21. In this embodiment, a screw rod 57 is screwed into a cap 56 at the upper end of an inner barrel 51, this screw rod being rotatably mounted to a sliding pipe 58 which may slidably fit with the outer circumference of the inner barrel 51 by means of a handle 59. In a state of FIG. 20, prior to increase of the height of vehicle, in which a rider rides and runs into the wasteland, for ease of understanding, let us assume that the inner barrel 51 stands still, a handle 59 is turned so that the sliding pipe 58 may upwardly be moved. At this time, the mounting length of the front fork is increased from L.sub.1 to L.sub.2 as the suspension spring is compressed to maximum extent. At this time, since the initially set load of the suspension spring 54 remains the same, the rider can experience the same feeling as when the height of vehicle is increased as when he runs on the flat urban area, but since the total stroke l.sub.t remains the same, a large thrust from below cannot be absorbed when he runs on the wasteland as in the above-described type in which the initially set load can be changed. In addition, since the mounting length of the front fork, which is compressed to maximum extension after the height of the vehicle is increased, becomes larger by a stroke of the sliding pipe 58 than before the height of vehicle is increased, a large space can become necessary between the vehicle body and the tire.
Accordingly, it is a principal object of the present invention to provide a novel shock absorber for use in a vehicle in which drawbacks inherent to the above-described two types are eliminated, i.e., the initially set load is also not changed as the height of vehicle is increased and the total stroke is made longer while at the same time the rebound stroke remains the same and yet the mounting length of the absorber assumed as compressed to maximum extent also remains the same.
Another object of the present invention is to provide a novel shock absorber having an apparatus for manually adjusting the height of vehicle for use in a vehicle, in which the initially set load of the suspension spring is not changed as the height of vehicle is increased while at the same time the stroke of piston is made longer and yet the mounting length of the shock absorber assumed as compressed to maximum extent is not also changed.
A further object of the present invention is to provide an inexpensive shock absorber having a hydraulic drive source for driving an apparatus for adjusting the height of vehicle for use in a vehicle, in which the initially set load of the suspension spring is not changed as the height of vehicle is increased while at the same time the stroke of piston is made longer and yet the mounting length of the shock absorber assumed as compressed to maximum extent is not also changed, said apparatus using a combination of a pressure tank and a check valve in place of an electromotive hydraulic pump.